The present invention relates to fluorescent lamps, and more particularly to a fluorescent lamp which is compact in size, has a high lumen output for its packaged dimensions, and is capable of being operated at a variable lumen output.
Compact fluorescent lamps found on the market today generally comprise a single fluorescent tube which has been folded several times allowing it to fit into a small space and therefore, giving this type of lamp access to the market which has previously been the exclusive domain of incandescent lamps. These compact fluorescent lamps are desirable replacements for incandescent lamps primarily because they provide an efficient light source having a range of power and light output in the area of 5 to 55 watts with 250 to 4800 lumens. Compact fluorescent lamps are therefore more efficient at converting electrical energy to visible light than incandescent lamps and typically deliver 50 to 60 lumens per watt, while the efficiency of an incandescent lamp is 16 lumens per watt. Thus, replacement of incandescent lamps with compact fluorescent lamps can yield substantial energy savings.
Furthermore, incandescent lamps have a lifetime of only about 750 hours while compact fluorescent lamps are capable of lasting from 6,000 to 10,000 hours. Additionally, in a commercial environment, where replacement of light bulbs incurs labor costs, the less often a bulb requires changing, the more economical the installation.
Electric utilities also desire to reduce the peak load demand by supporting energy conservation, as the construction of additional power generating facilities, or running less efficient auxiliary generators to accommodate a peak load, is costly. Utilities have therefore, found it highly economical to support energy saving devices such as compact fluorescent lamps to reduce peak load. This has given a large boost to the entire field of compact fluorescent lamp development.
Reducing the size of a compact fluorescent lamp in order to expand its applicability has been a goal of the lighting industry, however problems of thermal management become increasingly prohibitive as the dimensions of the lamp shrink. Compact fluorescent lamps generally comprise a glass envelope with a phosphor coating on its interior surface, each end of the lamp having an oxide coated electrode, the oxide coating serving to enhance electron emission. The glass envelope, during operation, has about six millitorr vapor pressure of mercury and several torr of a rare gas, e.g., argon, and a low pressure discharge is maintained between the two electrodes causing the mercury to emit visible and ultraviolet radiation, the ultraviolet radiation being converted to visible light by the phosphor coating. The performance of the lamp is therefore strongly dependent upon the mercury pressure in the lamp, which increases with temperature.
With a typical ambient temperature of 25.degree. C. some of the heat generated by the discharge beneficially warms the cold spot to the ideal temperature of about 40.degree. C. and at this temperature, the vapor pressure of mercury delivers the maximum ultraviolet radiation to the phosphor coated walls. Standard fluorescent lamps have been engineered to operate at the ideal temperature, however the domain of compact fluorescent lamps is in compact applications. Therefore, to attain the desired luminous flux from a compact fluorescent lamp, while maintaining its compactness, requires that the wall loading or power per unit surface area be increased over that from a standard fluorescent lamp. This causes the cold spot temperature of the compact fluorescent lamp to rise beyond the ideal of 40.degree. C., and the efficiency of the lamp drops.
There are basically two methods for solving this problem. A region of the glass envelope can be cooled by changing its geometry or by heat sinking it. Because the mercury vapor fills the entire volume of the glass envelope, cooling any small portion of that envelope will effectively control the mercury pressure anywhere in the lamp. This method has the disadvantage of constraining the possible geometries available to a lamp designer, and furthermore the application of the compact fluorescent lamp in a fixture may obviate advantages gained by altering the geometry.
The second solution is to use an amalgam of mercury and a metal such as indium which has a lower vapor pressure than mercury itself. Without such an amalgam, the efficiency of a compact fluorescent lamp is within 10% of its optimum over a narrow 25.degree. C. range centered at about 40.degree. C. With an amalgam, the range is shifted to higher temperatures, specifically tailored to those encountered in a compact fluorescent lamp, and the efficiency is within 10% of its optimum over a range of 40.degree. C.-120.degree. C. This makes the lamp both efficient at the nominal operating temperature of the lamp and makes it insensitive to departures from the specified operating temperature. However, the mercury pressure takes longer to become established because the lamp has a longer warm-up time, thus delaying the time at which the lamp attains its maximum light output.
An additional problem resulting from overheating is the degradation of the electronic ballast. The addition of an integral electronic ballast to compact fluorescent lamps expands their applicability, but it also thermally couples the lamp to the electronics. This and the compactness of the source, causes the temperature of the components to rise and shortens their useful life.
It is also found that as the size of the compact fluorescent lamp is reduced, the phosphor loading (power per unit surface area covered with phosphor) increases, leading to faster phosphor light output deterioration. This is due to the density of damaging species that impinge on the phosphor. Mercury ions tend to sputter the phosphor and implant themselves causing darkening, which inhibits the generation and transmittance of visible radiation. Radiation can also damage the phosphor, in particular, the 185.0 nm mercury radiation is somewhat damaging to the phosphor. Under normal operating conditions of the compact fluorescent lamp, that is a cold spot temperature of 40.degree. C., the amount of 185.0 nm radiation is only about 6%. However, as the temperature goes up this percentage increases to as much as 20-30% depending on the temperature. Phosphors have been improved so that they can withstand a higher wall loading, yet they remain a weak link in the longevity of compact fluorescent lamps. One method of protecting the phosphor is to coat it with a thin film of alumina the coating being transparent to ultraviolet radiation, allowing the ultraviolet radiation to strike the phosphor and generate visible light, while shielding the phosphor from damaging species.
Packaging therefore remains a problem for compact fluorescent lamps making them in many instances an unsuitable retrofit for many incandescent applications. They do not fit into many standard fixtures for incandescent applications which seriously hinders the use of compact fluorescent lamps as retrofit replacements for incandescent lamps.
It is therefore an object of the present invention to provide a compact fluorescent lamp which overcomes the various disadvantages of those compact fluorescent lamps of the prior art, and which is simple to manufacture, employing readily available components.
A further object of the present invention is to provide a compact fluorescent lamp which the light output can be varied during operation.
Yet another object of the invention is to provide a compact fluorescent lamp which may be constructed from currently manufactured components and operated from a single ballast for simplicity.
Yet another object of the present invention is to provide a configuration for arc tubes that utilizes space in an optimum fashion without necessitating the development of new manufacturing equipment for making the lamps.